Читать книгу High-Performance Materials from Bio-based Feedstocks - Группа авторов - Страница 70

Milescu et al. [34] have published data on the adsorption of three toxic gases on starch, alginic acid, and pectin‐based materials and found some very interesting behaviour. The authors studied the room temperature adsorption of the gases from 5000 ppm in air mixtures, and looked at ammonia, hydrogen sulphide, and sulphur dioxide (separately). They found excellent adsorption in many cases, with materials outperforming standard activated carbons such as Norit, and also giving impressive results compared with a range of other adsorbents discussed in the literature.

For ammonia, adsorption was best with low‐temperature‐activated alginic acid material (in particular, the uncarbonised A000) with performance dropping to very low levels with the higher temperature materials. This correlates very well with the availability of acidic functionality (via the alginic acid‐repeating units) which is highest at carbonisation temperatures <200 °C. This functionality is lost above this temperature, and with it, the ability to adsorb ammonia drops dramatically. For both the starch and the pectin‐derived materials, there is no inherent acidity and the best performance for these materials is seen at 300 °C activation. This activity likely correlates well with a high level of functionality, including aldehydic/ketonic groups formed by dehydration of vicinal diols, and also possibly by ring scission. Such groups will readily form imines, trapping the nitrogen. Evidence for nitrile formation is also strong, possibly via dehydration of primary amides, from acidic functionality that can also develop during carbonisation. Further carbonisation leads to the gradual loss of this rich functionality, leading to a reduced ability to adsorb ammonia.

For both the sulphur‐containing gases, adsorption ability was dominated by the highest temperature materials (in this case, 800 °C was the maximum used). Indeed, for hydrogen sulphide, the pectin‐derived materials were exceptionally active, adsorbing 20 times as much as any other material tested, and approximately 4 times as much as a pectin‐derived P550 material. Interestingly, XPS studies on the loaded samples showed that, while most of the sulphur loaded was in the original oxidation state, there were significant quantities of oxidised sulphur species present (S(IV) and S(VI)). Small amounts of reduced sulphur species were noted on SO₂ adsorption, along with more prevalent oxidised species. The presence of high oxidation state N species in very low quantities on the surface of the materials was suggested as a potential catalytic route to oxidation of the S species. The significant amounts of inorganics in the materials may also play a role, especially in the high‐temperature materials, where the inorganics are concentrated via loss of organics – S800 has only 2.6% inorganics, A800 a significant 8.8%, and P800 (from pectin) a huge 28.6%, much of it potassium.

Dura et al. published results relating to the impressive ability of Starbon to adsorb carbon dioxide, which is an important topic for control of CO₂ levels and also to concentrate CO₂ prior to its conversion into valuable chemicals such as cyclic carbonates [35]. The authors compared the predominantly mesoporous Starbon (68–92% of the total pore volume was in the mesopore range) with Norit‐activated carbon which is c. 75% microporous. They utilised both starch‐based and alginic‐acid‐derived materials in their study, and these were prepared over a wide temperature range from 300 to 1200 °C.

Pressure swing adsorption data were collected over 5 cycles at 10 bar pressure. This demonstrated that, in both the starch‐ and alginic‐acid‐derived materials, CO₂ adsorption increased from low levels for the S300 and A300, reaching a maximum at the S800 and A800 materials. Beyond the 800 materials, there was a slight drop in the case of the starch materials, but a substantial reduction in adsorption in the alginic acid series. The optimal adsorption capacity was 40% (S800) and 50% greater (A800) than for Norit. Adsorption kinetics were the same for all three adsorbents, with saturation after 30 minutes at 5 bar pressure or after 10 minutes for 10 bar pressure. Desorption under atmospheric pressure took 20 minutes in all cases.

Simultaneous thermal analysis was used to measure adsorption/desorption under flowing conditions alternating between CO₂ and nitrogen. This indicated rapid and complete reversibility of adsorption over many cycles, as well as allowing estimation of the enthalpy of adsorption. The values (between −14 and −18 kJ mol⁻¹) indicate that physisorption is the predominant mechanism. Regression analysis was used to develop an empirical equation to correlate the adsorption to textural properties and led to the equation:

In other words, the adsorption behaviour is strongly linked both to micropore adsorption and to a combination of micropores and mesopores, suggesting that the combination of mesopores and micropores allows excellent transport to the adsorption sites as well as some mesopore adsorption.

Very importantly, CO₂ to nitrogen selectivity was measured for Norit, S800 and A800 using mixed gas streams at 298 and 323 K. What was striking here was that selectivity to CO₂ was much higher for the Starbon materials (14.0–20.3) than for Norit (5.4 at 298 K and 4.0 at 323 K) meaning that adsorption in real situations would benefit from the use of the Starbon materials.